Add missing branch explanations for while-loop

2 files changed, 20 insertions, 5 deletions

diff --git a/coverage/parser.py b/coverage/parser.py
index 84cd3d1d..f84133d2 100644
--- a/coverage/parser.py
+++ b/coverage/parser.py
@@ -870,19 +870,21 @@ class AstArcAnalyzer(object):
         if constant_test:
             to_top = self.line_for_node(node.body[0])
         self.block_stack.append(LoopBlock(start=start))
-        exits = self.add_body_arcs(node.body, from_start=ArcStart(start))
+        from_start = ArcStart(start, cause="the condition on line {lineno} was never true")
+        exits = self.add_body_arcs(node.body, from_start=from_start)
         for xit in exits:
-            self.add_arc(xit.lineno, to_top)
+            self.add_arc(xit.lineno, to_top, xit.cause)
         exits = set()
         my_block = self.block_stack.pop()
         exits.update(my_block.break_exits)
+        from_start = ArcStart(start, cause="the condition on line {lineno} was never false")
         if node.orelse:
-            else_exits = self.add_body_arcs(node.orelse, from_start=ArcStart(start))
+            else_exits = self.add_body_arcs(node.orelse, from_start=from_start)
             exits |= else_exits
         else:
             # No `else` clause: you can exit from the start.
             if not constant_test:
-                exits.add(ArcStart(start))
+                exits.add(from_start)
         return exits
 
     @contract(returns='ArcStarts')
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 0d00409e..1be5e16c 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -206,8 +206,13 @@ class ParserMissingArcDescriptionTest(CoverageTest):
 
             def func5():
                 for x in range(6):
-                    if x == 3:
+                    if x == 7:
                         break
+
+            def func10():
+                while something(11):
+                    thing(12)
+                more_stuff(13)
             """)
         self.assertEqual(
             parser.missing_arc_description(1, 2),
@@ -225,6 +230,14 @@ class ParserMissingArcDescriptionTest(CoverageTest):
             parser.missing_arc_description(6, 7),
             "line 6 didn't jump to line 7, because the loop on line 6 never started"
         )
+        self.assertEqual(
+            parser.missing_arc_description(11, 12),
+            "line 11 didn't jump to line 12, because the condition on line 11 was never true"
+        )
+        self.assertEqual(
+            parser.missing_arc_description(11, 13),
+            "line 11 didn't jump to line 13, because the condition on line 11 was never false"
+        )
 
     def test_missing_arc_descriptions_for_small_callables(self):
         # We use 2.7 features here, so just skip this test on 2.6